A typical configuration of an HVDC power transmission system has two converter stations that are linked by DC transmission lines. Each converter station employs an AC/DC converter to connect the DC transmission lines to an AC network or power grid There are two different types of HVDC power converter technologies, namely, Current Sourced Converters (CSC) and Voltage Sourced Converters (VSC). VSC HVDC systems are the latest technology, and can easily be used: (a) to construct a Multi-terminal HVDC system; (b) for bulk power transmission and system interconnection; (c) for large scale renewable generation integration; and (d) to construct hybrid AC/DC systems, etc.
Examples of commercially available systems using VSC converters include two- or three-level converter topologies and a Multi-level Modular Converter (MMC) topology as well as a cascaded two-level converter topology. Also, for these converter technologies, there are different variants such as half bridge MMC topology, full bridge MMC topology as well as hybrid converter topologies. The hybrid converter topologies, which combine the features and advantages of both of the MMC and 2-level converters, can be formed from a combination of high-voltage series switches, using IGBTs connected in series, and “wave-shaping” circuits based on the same types of “half-bridge” and “full-bridge” cell which make up the MMC. The wave-shaping circuits can be connected either in series or in parallel with the series-switch circuits.
FIGS. 1a and 1b show the main structures of a one-terminal MMC VSC of a HVDC system. As shown in FIG. 1 a, power is supplied to or from an AC system 1. There is a point of common coupling (FCC) 2 of the buses of the VSC converter station, which is connected to a transformer 3 having a star-delta configuration. Power to/from the transformer 3 is provided from/to a MMC 8, via a line that includes three-phase AC shunt filters 4, having a resistance and inductance represented by resistor 5 and inductor 6, and an AC connector 7. On the DC side of the converter are poles P1 and P2 with capacitors 9 and 10 between the pores and earth. P1 is connected to DC terminal pole P3 with the reactance of the line represented by reactance 11, while P2 is connected to DC terminal pole P4 with the reactance of the line represented by reactance 12.
FIG. 1b shows the structure of the MMC converter 8. In this example, there are three bridges 18, 19, 20 for three phases, respectively, in each converter. There are two arms, 18a, 18b; 19a, 19b; 20a, 20b in each bridge. There are sub-modules 13 in each arm, and for an n-level MMC HVDC system there will be n−1 sub-modules 13 in each arm. Each arm 18a, 18b; 19a, 19b; 20a, 20b also has a reactor 14 used to facilitate current control within the phase arms and limit fault currents.
DC fault current suppression is extremely important for a two-terminal or Multi-terminal VSC HVDC power transmission system to suppress both AC and DC currents arising from a DC-side (or DC-grid) short circuit, to control and protect the converters and the DC grid, and hence isolate the faulty DC circuit. Such a control and protection strategy is crucial to all types of MMC VSC HVDC Systems and their variants, in particular for a Multi-terminal configuration.
A full bridge MMC topology can suppress a DC fault current, but needs more semiconductor (IGBT) modules and produces higher power losses. The “full-bridge” MMC VSC can reverse the voltage to counteract the AC side voltage, and in this way the converter bridge can totally block the current flow, thereby suppressing fault currents arising from DC-side short circuit events by the converter control action alone. However, the full bridge MMC topology requires, in principle, twice the number of IGBTs compared with a half bridge MMC of the same rating, resulting in an increase in costs and power losses. Thus, the half-bridge MMC is favoured from the point of view of the economics of the MMC itself. Unfortunately, as well as with a 2-level VSC system, a 3-level VSC system and cascaded 2-level or 3-level systems, currently half bridge MMC VSCs do not possess the ability to suppress fault currents arising from DC-side short circuit events by converter control action. As a consequence, half bridge MMC VSC systems are required to include expensive DC circuit breakers, thereby substantially eroding their economic benefits.
It would thus be desirable to be able to use a half bridge VSC MMC in a two-terminal or Multi-terminal HVDC power transmission system where a DC fault current can be controlled or managed by the action of the VSC converter itself.